The present invention generally relates to structures subject to high stresses and high temperatures, such as rotating components of gas turbines and other turbomachinery. More particularly, this invention relates to a method of inhibiting heat transfer to angel wings of turbine buckets (blades) so as to reduce the temperature of the angel wings and/or reduce the cooling requirements of the angel wings.
Buckets (blades), nozzles (vanes), and other components located in the hot gas path within turbine sections of gas turbines are typically formed of nickel-, cobalt- or iron-base superalloys with desirable mechanical and environmental properties for turbine operating temperatures and conditions. Because the efficiency of a gas turbine is dependent on its operating temperatures, there is a demand for components that are capable of withstanding increasingly higher temperatures. As the maximum local temperature of a component approaches the melting temperature of its alloy, forced air cooling becomes necessary. For this reason, airfoils of gas turbine buckets and nozzles often require complex cooling schemes in which air is forced through internal cooling passages within the airfoil and then discharged through cooling holes at the airfoil surface.
FIG. 1 schematically represents an axial cross-section of a turbine section 10 of a land-based gas turbine engine. The turbine section 10 comprises multiple turbine stages, represented as the first and second stages immediately downstream of the combustor (not shown) of the turbine engine. Each stage of the turbine section 10 comprises an annular array of circumferentially-spaced buckets 12 (only one bucket 12 of each stage is represented in FIG. 1) and a nozzle assembly 14 made up of an annular array of circumferentially-spaced vanes 16 (only one vane 16 of each stage is represented in FIG. 1). The nozzle assemblies 14 and their vanes 16 are statically mounted within the turbine section 10, whereas the buckets 12 are mounted on a rotating component, commonly referred to as a wheel 18, of the gas turbine to enable rotation of the buckets 12 within the gas turbine and relative to the nozzle assemblies 14. The vanes 16 define airfoils that extend between inner and outer platforms (or bands) 20 of the nozzle assemblies 14. As represented in FIG. 1, each bucket 12 comprises an airfoil 24 extending from a shank 26 in a radially outward direction 22 of the turbine section 10. The buckets 12 can be conventionally anchored to their respective wheels 18, for example, with dovetails (not shown) formed on their shanks 16 and received in complementary slots defined in the circumference of each wheel 18. The buckets 12 and nozzle assemblies 14 are directly subjected to the hot gas path 32 within the turbine section 10. In particular, the airfoils 24 of the buckets 12 and the vanes 16 of the nozzle assemblies 14 are impinged by the hot combustion gases in the hot gas path 32 through the gas turbine.
Impingement of the bucket airfoils 24 and nozzle vanes 16 by the combustion gases results in upstream airfoil wakes and downstream airfoil bow waves, which tend to produce pressure wakes within the hot gas path 32 that cause hot combustion gases to be driven into trench cavities 34 between rows of buckets 12 and nozzle assemblies 14 and, from there, into wheelspace cavities 36 between the wheels 18. To inhibit the ingress of hot combustion gases into the interior regions of the gas turbine, the buckets 12 are commonly equipped with extensions, referred to as angel wings 28, that extend from the shank 26 into the trench cavities 34 in a direction corresponding to the axial direction of the turbine section 10. As represented in FIG. 1, the angel wings 28 cooperate with lands 30 formed on the adjacent nozzle assemblies 14 to create a tortuous path that inhibits the flow of hot gases through the trench cavities 34. Consequently, the angel wings 28 are directly exposed to the hot combustion gases ingested into the trench cavities 34 from the gas path 32. Current practice is to supply the trench cavities 34 with a cooling air flow 33 obtained by air bled from the compressor section (not shown) of the engine for the purpose of keeping the angel wings 28 at temperatures that are sufficiently low to enable the angel wings 28 to meet their creep and fatigue life requirements. However, this purge flow is costly to the overall performance of a gas turbine engine, and therefore any reduction in the cooling air flow 33 needed to protect the angel wings 28 would be advantageous to turbine efficiency.